The petroleum industry uses many different methods of in situ stimulation of heavy oil and bitumen present in various carbonate and sandstone reservoirs where the oil is too deeply buried to be mined. In many reservoirs, the heavy oil or bitumen is so viscous that it needs to be warmed in order to flow at economic rates. Steam Assisted Gravity Drainage (SAGD) as described in U.S. Pat. No. 4,344,485 (Butler, Aug. 17, 1982), Cyclic Steam Stimulation (CSS) or “huff and puff”, In situ Combustion, Waterflooding, Miscible carbon dioxide enhanced oil recovery (CO2-EOR), vapor-assisted petroleum extraction (VAPEX), and Downhole Heaters are some of the more common methods. Current drilling methods for drilling wells useful for in situ stimulation and production of heavy oil/bitumen generally use a conventional, single wall drill string that uses a conventional or underbalanced mud system.
Conventional drilling methods using single wall drill string require that the drill cuttings and mud be returned to surface on the outside of the single wall drill string. In certain reservoirs, using single wall drill string can result in formation damage and serious lost circulation problems. Lost circulation is loss of substantial quantities of drilling mud to an encountered formation during borehole drilling. This is evidenced by a total or drastic reduction of returning mud and a reduction in the volume of mud in the mud pits. The following could cause lost circulation: borehole pressure (mud pressure) being in excess of the formation pressure; damaged formations due to reckless drilling; pipe surging at high speeds; fractured, fissured or faulted formations; limestone regions, which are vuggy and very coarse; permeable rocks like pebbles, reefs and irregular limestone, gravels and conglomerates.
The undesirable effects of lost circulation include: loss of drilling energy; sudden undesirable speed increase of the rotary; deflection of the bit along joint planes or even breaking of the bit; drilling fluid may be totally lost, hence increased cost of operation; time wasted in pulling back and/or combating lost circulation; drop in annular level may cause blow out in over-pressured or gas-bearing formations; loss of information from the down-hole; and the chances of stuck-pipe and fishing exercise are increased, if lost circulation occurs in an aquiferous zone, or slightly above it, then completion and development of the borehole may be impaired.
When drilling in formations such as oil sand and oil shale, damage to the formation may also occur when drilling back up the hole to remove the drill string. Hence, removal of the drilling apparatus from the drilled hole may also result in lost circulation. Thus, it would be desirable at the very least to drill and complete a well without having to remove the drill string after drilling the borehole.
Furthermore, borehole cleaning in heavy oil and bitumen reservoirs is major problem and requires additional drilling time and money and may result in increased formation damage. Running production casing or a slotted liner may be very difficult when the well bore hasn't been properly cleaned.
The present application uses both single wall drill string and dual wall (concentric) drill string that can remain downhole to now operate as a production well, a stimulation well or both. By eliminating the need to drill back up the hole, the likelihood of lost circulation can be reduced.
Use of dual wall drill pipe or dual wall coiled tubing to drill the well will further reduce drilling damage and lost circulation problems. Hole cleaning is much easier and more effective when using dual wall drill string, as the drill mud and cuttings travel up the inside tube. This avoids contact with the formation and agents such as chemicals and foam can be added to assist in borehole cleaning by delivering them through the annulus formed between the inner and outer tubes of the concentric drill string
The method and apparatus as described in the present application can also be used to produce and/or stimulate the flow of heavy oil/bitumen, either alone or in combination with other well stimulation techniques known in the art.